In an optical transmission system, SDH (Synchronous Digital Hierarchy) is internationally standardized as a digital hierarchy for multiplexing existing service signals. In the United State, SONET (Synchronous Optical Network) similar to SDH is a standard.
Regarding current optical transmission systems, optical transmission systems complying with SONET/SDH specifications are dominating, and the optical transmission systems have been introduced in large quantities in the world so far. In recent years, being based on wavelength-division multiplexing (WDM) that can support explosive increase of the Internet traffic, Optical Transport Network (OTN) (refer to non-patent document 1, for example) is standardized as a platform that transparently transmits various client signals which are not only SDH/SONET but also ATM and Ethernet (registered trademark) and the like, and it is expected that the Optical Transport Network (OTN) will become the mainstream of optical transmission systems in the future.
Client signals of SONET/SDH and OTN are predicated on SONET/SDH signals. As bit rates of client signals of OTN, 2.488, 9.953, and 39.813 Gbit/s are defined in which the number increases in multiples of four.
However, bit rates of Ethernet signals which are being explosively widespread in recent years do not increase in multiples of four. In addition, the bit rates do not increase in multiples of an integral number. The signal of 1 Gigabit Ethernet (1 GbE) is 1.25 Gbit/s, and 10 Gigabit Ethernet (10 GbE) LAN-PHY signal is 10.3125 Gbit/s.
In the future, it is expected that 10 GbE becomes the mainstream as client signals of communication carriers. In addition, demands for connecting LAN environments, as they are, scattered over remote locations by using LAN-PHY are increasing. Further, as to the Ethernet signal, there are users and device vendors using the preamble and the inter-frame gap (IFG) based on proprietary specifications that are different from the IEEE standard (refer to non-patent document 2, for example).
Therefore, in carrier networks, for some Ethernet signals in which the bit rates are not integral multiples of each other, it is required to transparently transfer the whole of the signal including the preamble and the IFG other than the frame.
As a technique for transparently accommodating high speed data signals such as GbE into the SONET/SDH or the OTN network to transmit the signals, there is a virtual concatenation technique that can flexibly change bands. For realizing high reliability also for the high speed data signals of the Gbit/s class, a hitless switching apparatus that can support high speed signals using virtual concatenation is actually used (non-patent document 3, for example).
FIG. 1 shows an example of a transmission system of a conventional technology. The client signals are distributed into SDH virtual containers (VC-3/4) each of which forms a multi-frame using H4 byte in the SDH path overhead. Each VC signal is branched off by the transmission unit 1. The branched VC signals are transmitted by an active transmission route and an auxiliary transmission route. The reception unit 2 performs delay adjustment for signals using the multi-frame in the active route and the auxiliary route so as to select one of them.
Further, in the reception side, the order of VC signals are recognized by using multi-frame counter values to recover the original client signal. In addition, in the transmission system shown in FIG. 1, by accommodating the SDH signal generated using the virtual concatenation into OTN, it becomes possible to transfer the signals by OTN. Accordingly, high speed signals including 10 GbE-LAN-PHY can be flexibly accommodated so as to be transparently transferred.
[Non Patent document 1] ITU-T G.709
[Non Patent document 2] Haruhiko Ichino, Kazuhiko Terada, Kenji Kawai, Osamu Ishida, Keishi Kishine, Noboru Iwasaki, “ENHANCED NETWORK SIGNALING FOR 10 GIGABIT ETHERNET TO ACHIEVE A LAN-WAN SEAMLESS INTERFACE AND ITS IMPLEMENTATION IN THE PHY-LSI/TRANSCEIVER MODULE”, International Journal of High-Speed Electronics and Systems, Vol. 15, No. 3 (2005)667-704, World Scientific Publishing Company
[Non Patent document 3] ATSUSHI SUDO, KAZUHIRO ODA, IEICE general conference, B-10-67, 2002